This invention relates to a masking device and a method therewith for defining a high resolution thin film pattern on a flat substrate by vacuum deposition or sputtering.
Most commonly used methods for generating high resolution thin film patterns on substrates make use of physical barriers, or masks, which are in close proximity to the substrate during film deposition. Apertures cut or etched into the mask allow portions of the evaporant beam to intercept and condensate upon the exposed regions of the substrate.
In order to produce sharply delineated film patterns, it is necessary that the mean free path of the evaporant particles be long compared to the mask to substrate spacing, and that the sticking coefficient of the evaporant beam be close to unity to prevent seepage under the mask. If the mask is in physical contact with the substrate, and the substrate is maintained at a relatively low temperature, it is possible to achieve good results. This is not the case with sputtering, however, because of the higher gas pressure and temperature. Physical contact is particularly important in sputtering systems since the evaporant particles are imparted considerable kinetic energy, allowing them to penetrate under the prior art masks. Therefore, sharp film patterns have not been obtainable by sputtering.
In the past, masks of metal foil, graphite, and glass have been used. These, however, have suffered from a number of problems owing to the materials themselves and the methods used in their manufacture. Patterns smaller than a few thousandths of an inch cannot be achieved accurately and repeatably, and because these masks must be thin to accommodate high resolution patterns they are typically not stiff enough to maintain good physical contact with the substrate. Furthermore, where thicker masks are used, machining and etching techniques have produced apertures having side walls substantially normal to the face of the mask, allowing deposited material to accumulate in and clog the aperture without contacting the substrate. See generally, Maissel & Glang, Handbook of Thin Film Technology, McGraw-Hill 7-1 to 7-7 (1970).
In addition, cleaning techniques capable of removing the evaporated species from a mask generally attack the mask itself, rendering it unusable. U.S. Pat. No. 4,391,034 (1983), issued to Stuby, suggests the use of a molybdenum mask to satisfy a long felt need in the art for reusable masks. While molybdenum may be hardier than other metals, and therefore more suitable for sputtering applications, its use still limits pattern resolution. Stuby also recognizes that masks and substrates generally have different coefficients of thermal expansion.
U S. Pat. No. 4,256,532 (1981), issued to Magdo et al., proposes a silicon shadow mask for use with silicon substrates to overcome thermal expansion problems during deposition. Magdo's apertures are etched in such a way as to purposefully limit lateral growth, resulting in substantially vertical side walls. In addition, the resulting slightly tapered aperture is oriented so that its smaller cross-section faces the evaporant beam, not the substrate. While this may assist the user in lifting-off the mask after deposition, it frustrates the attainment of maximum resolution which is of primary importance to the users of masks, and it does not solve the problems associated with aperture clogging and seepage under the mask.